Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a de facto standard communication protocol, and it is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are:
- It has been developed with industrial applications in mind
- It is openly published and royalty-free
- It is easy to deploy and maintain
- It moves raw bits or words without placing many restrictions on vendors
Modbus allows for communication between many (approximately 240) devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.
The development and update of Modbus protocols has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to that organization, signaling a clear commitment to openness. The Modbus Organization is an association formed of independent users and suppliers of Modbus compliant devices that seeks to drive the adoption of the Modbus communication protocol suite, and its evolution to address architectures for distributed automation systems across multiple market segments.
Versions of the Modbus protocol exist for serial port and for Ethernet and other networks that support the Internet protocol suite. There are many variants of Modbus protocols:
- Modbus RTU — This is used in serial communication & makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods.
- Modbus ASCII — This is used in serial communication & makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon (':') and trailing newline (CR/LF).
- Modbus TCP/IP or Modbus TCP — This is a Modbus variant used for communications over TCP/IP networks, connecting over port 502. It does not require a checksum calculation as lower layers already provide checksum protection.
- Modbus over TCP/IP or Modbus over TCP or Modbus RTU/IP — This is a Modbus variant that differs from Modbus TCP in that a checksum is included in the payload as with Modbus RTU.
- Modbus over UDP — Some have experimented with using Modbus over UDP on IP networks, which removes the overheads required for TCP
- Modbus Plus (Modbus+, MB+ or MBP) Modbus over Fieldbus (Modbus+ or MB+), also exists, but remains proprietary to Schneider Electric. requires a dedicated co-processor to handle fast HDLC-like token rotation. It uses twisted pair at 1 Mbit/s and includes transformer isolation at each node, which makes it transition/edge triggered instead of voltage/level triggered. Special interfaces are required to connect Modbus Plus to a computer, typically a card made for the ISA (SA85), PCI or PCMCIA bus.
- Modbus PEMEX- This variant is an extension of standard Modbus with support for historical and flow data. It was designed for process control and never gained widespread adoption
- Enron Modbus- This variant is an extension of standard Modbus with support for 32 bit Integer and Floating Point variables, and historical and flow data. Data types are mapped using standard addresses. The historical data is used to meet an American Petroleum Institute (API) industry standard for how data should be stored[
Data model and function calls are identical for the first 4 variants of protocols; only the encapsulation is different. However the variants are not interoperable as the frame formats are different.
Communication and devices
Each device intended to communicate using Modbus is given a unique address. In serial and MB+ networks only the node assigned as the Master may initiate a command, but on Ethernet, any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcastable commands sent to node 0 which are acted on but not acknowledged). All Modbus commands contain checking information, ensuring that a command arrives undamaged. The basic Modbus commands can instruct an RTU to change a value in one of its registers, control or read an I/O port, as well as commanding the device to send back one or more values contained in its registers.
There are many modems and gateways that support Modbus, as it is a very simple protocol and often copied. Some of them were specifically designed for this protocol. Different implementations use wireline, wireless communication, such as in the ISM band, and even SMS or GPRS. One of the more common designs of wireless networks makes use of the mesh topology. Typical problems the designers have to overcome include high latency and timing problems.
All Modbus variants choose different frame formats.
|Modbus RTU Frame Format|
|Start||3.5c idle||at least 3-1/2 character times of silence (MARK condition)|
|Address||8 bits||Station Address|
|Function||8 bits||Indicates the function codes like read coils / inputs|
|Data||n * 8 bits||Data + length will be filled depending on the message type|
|CRC||16 bits||Error checks|
|End||3.5c idle||at least 3-1/2 character times of silence between frames|
|Modbus ASCII Frame Format|
|Start||1 char||starts with colon ( : ) (ASCII value is 0x3A)|
|Address||2 chars||Station Address|
|Function||2 chars||Indicates the function codes like read coils / inputs|
|Data||n chars||Data +length will be filled depending on the message type|
|LRC||2 chars||Error checks|
|End||2 chars||carriage return – line feed(CR/LF) pair (ASCII values of 0x0D & 0x0A)|
|Modbus TCP Frame Format|
|Transaction Identifier||2 bytes||For synchronization between messages of server
|Protocol Identifier||2 bytes||Zero for Modbus/TCP|
|Length Field||2 bytes||Number of remaining bytes in this frame|
|Unit Identifier||1 byte||Slave Address (255 if not used)|
|Function code||1 byte||Function codes as in other variants|
|Data bytes||n bytes||Data as response or commands|
Unit identifier is used with Modbus/TCP devices that are composites of several Modbus devices, e.g. on Modbus/TCP to Modbus RTU gateways. In such case, the unit identifier tells the Slave Address of the device behind the gateway. Natively Modbus/TCP-capable devices usually ignore the Unit Identifier.
The byte order is Big-Endian (first byte contains MSB).
Note: The "Function code" field is part of the PDU and not part of the transport (TCP) header.
Supported function codes
The various reading, writing and other operations are categorised as follows.The most primitive reads and writes are shown in bold. A number of sources use alternative terminology, for example Force Single Coil where the standard uses Write Single Coil.
|Function Name||Function Code|
|Data Access||Bit access||Physical Discrete Inputs||Read Discrete Inputs||2|
|Internal Bits or Physical Coils||Read Coils||1|
|Write Single Coil||5|
|Write Multiple Coils||15|
|16-bit access||Physical Input Registers||Read Input Register||4|
|Internal Registers or Physical Output Registers||Read Holding Registers||3|
|Write Single Register||6|
|Write Multiple Registers||16|
|Read/Write Multiple Registers||23|
|Mask Write Register||22|
|Read FIFO Queue||24|
|File Record Access||Read File Record||20|
|Write File Record||21|
|Diagnostics||Read Exception Status||7|
|Get Com Event Counter||11|
|Get Com Event Log||12|
|Report Slave ID||17|
|Read Device Identification||43|
|Other||Encapsulated Interface Transport||43|
Almost all implementations have variations from the official standard. Different varieties might not communicate correctly between equipment of different suppliers. Some of the most common variations are:
- Data types
- Floating point IEEE
- 32-bit integer
- 8-bit data
- Mixed data types
- Bit fields in integers
- Multipliers to change data to/from integer. 10, 100, 1000, 256 ...
- Protocol extensions
- 16-bit slave addresses
- 32-bit data size (1 address = 32 bits of data returned.)
- Word swapped data
- Since Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported.
- No standard way exists for a node to find the description of a data object, for example, to determine if a register value represents a temperature between 30 and 175 degrees.
- Since Modbus is a master/slave protocol, there is no way for a field device to "report by exception" (except over Ethernet TCP/IP, called open-mbus)- the master node must routinely poll each field device, and look for changes in the data. This consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link.
- Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station (once again Ethernet TCP/IP proving the exception).
- Modbus transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the transmission.
- Modbus protocol provides no security against unauthorized commands or interception of data.